In general, many of the tire-constituting members have a sheet-like shape, and end portions of such sheets are spliced to each other to assemble the tire-constituting members. Joining methods for a splice part include: an overlap method of overlapping end portions of a tire-constituting member one above the other; and a butt method of butting two end portions of a tire-constituting member.
Out of these tire-constituting members, an inner liner layer and a carcass layer are each arranged in a tire circumferential direction on a tire-forming drum, and are each formed by splicing their two end portions in the tire circumferential direction to each other. Therefore, the splice part of this formed body is subjected to a large shearing force in the tire circumferential direction during a lifting step in a tire forming process for expanding the diameter of the formed body, and during a lifting step in a curing process for expanding the diameter of an uncured tire in a curing mold. With this taken into consideration, the overlap method of overlapping end portions 1a of a tire-constituting member 1 one above the other as shown in FIG. 6 is used for the splice part of each of the inner layer and the carcass layer. A width x of the overlap between these end portions 1a is set large enough to fully absorb the amount of shift occurring between the end portions 1a due to a shearing force.
If this overlap width x is small in a heavy-duty pneumatic tire needing to keep its filled air pressure high and receiving a large load at its shoulder part, a splice part 2 of a carcass member in the shoulder part opens up, and an inner liner rubber and a tie rubber, which are arranged on an inner periphery of the carcass layer, go into the splice part 2. This causes what is termed as blowout failure, which later leads to fracture of the tread rubber.
If, however, the width x of the overlap between the two end portions 1a is made too large for the purpose of securing bonding strength of the splice part 2, the rigidity of the splice part 2 becomes locally large. As a result, vibrations of a tire while it is rotating are amplified, and accordingly the heterogeneity (uniformity) of the tire is deteriorated. For this reason, it has been an important issue that how large the width x of the overlap between the two end portions 1a should be set in order to achieve both the durability and the uniformity.
For the purpose of solving this problem, a proposal has been made in which: an ear rubber whose thickness is almost half of the thickness of a tire-constituting member, such as a carcass layer, is beforehand formed in each of end portions thereof in such a way as to project from the end portion; and the end portions are spliced to each other by overlapping one above the other (refer to Patent Document 1, for instance). However, this proposed method needs a step of forming the ear rubber in each of the end edges, and accordingly reduces its productivity. In addition, ear rubbers are likely to cause buckling during the splicing work because of their thinness; therefore, there is a problem that air gets into the splice part and accordingly the durability is decreased.
Patent Document 1: Japanese patent application Kokai publication No. 2001-322403